Several methods are known in the art to achieve impact modification of vinyl aromatic homopolymers and copolymers. Each method has some disadvantage. For example, it is known to dissolve an elastomeric polymer (elastomer) in a vinyl monomer mixture and to polymerize the mixture in the presence of the dissolved elastomer. Commercial high-impact polystyrene (HIPS) and some impact modified acrylonitrile-styrene-butadiene (ABS) resins are prepared by this method. This method affords modifiers having grafted rubbery particles of broad size distribution in the range of 1 to 5 micron (1000-5000 nanometers, nm) average particle diameter. Some workers consider such relatively large particle size modifiers to be necessary to afford the best impact properties in aromatic polymer blends, however, particle sizes greater than about 400 nm are highly detrimental to clarity of the blends, due to the sensitivity of visible light scattering to particle size in this particle size range.
To achieve clear formulations based on the bulk elastomeric polymer technology, acrylic/vinyl aromatic copolymers usually are polymerized in the presence of the elastomer, with monomers chosen to match the index of refraction of the elastomer (rubber) phase. This type of processing generally goes through a phase-inversion stage leading to a broad distribution of rubber particle sizes, or of domain (groups of particles) sizes, and consequently poor optical clarity. An example of this technology, such as by Jung (U.S. Pat. No. 4,308,354), describes typical rubber domain size distributions of 200-1,500 nm. Some examples (such as by A. Echte, et. al., in U.S. Pat. No. 4,330,641) require large domain size (equal to or greater than 3,500 nm) and high rubber levels (&gt;28 wt. %, based on total blend weight) to achieve good impact properties. Another reference (M. Starzak, B. Motysia, and M. Durak, Polyimery (Warsaw), 20, 596 (1975); CA: 85(8) 47,377Y) describes a process in which a "powdered butadiene rubber" is suspended in monomer and the monomer polymerized. After polymerization, the mixture is converted to an aqueous suspension. This type of processing also would be expected to give large rubber domains or agglomerates, and consequently poor optical properties.
In order to use a rubber-based modifier particle of smaller particle size (for example, less than about 400 nm.) for good optical clarity while still maintaining good impact values, many workers found it necessary to incorporate acrylonitrile, or other nitrile-containing vinyl comonomers (for example in clear ABS blends or formulations). However, acrylonitrile (AN) use is known to increase inherent yellowness and heat-aging yellowing properties of a blend to such a point as to be an undesirable solution when clarity and low yellowing are the sought properties. A representative example of this problem is described in U.S. Pat. No. 3,900,528 (L. Beer) which discloses AN/vinyl aromatic matrices modified with an AN-containing, 2-stage modifier having a methacrylate/butadiene/styrene (MBS) core, giving blends which typically show ASTM yellowness indices of about 20-50. Similarly, U.S. Pat. No. 4,046,839 (S. Papetti), discloses clear ABS formulations prepared by polymerizing styrene/acrylonitrile in the presence of a butadiene (Bd) latex in a process which converts the Bd latex to a suspension. In that process, polymerization of the monomer produces both a matrix polymer and an outerstage, or outer shell, on the Bd core. Since the comonomers are present in great excess over the number of rubber modifier particles, very high levels of outer shell are obtained on the rubber modifier particles. Accordingly, AN is incorporated into both matrix and modifier polymer and again results in a yellowness problem.
Attempts to make low-yellowness, impact modified, acrylic/vinyl aromatic polymer blends without the use of AN or other nitrile-containing monomers or polymers, while simultaneously maintaining a small, uniform particle size rubber impact modifier (for example, a latex-derived particle), are also known in the art. For example, F. Carrock and K. Chu (U.S. Pat. No. 3,887,652) disclose a process in which a monomer mixture is first grafted to a Bd rubber latex particle, then suspending agent is added, and more monomer is added which, after polymerization, converts the latex to a suspension bead product. This type of process, agglomerating the modifier particle into larger aggregates, gives improved impact but poor optical clarity. Another reference (J. Schmitt, U.S. Pat. No. 4,228,256) discloses a process for sequentially "outerstaging" a Bd latex particle with, for example, 5% methyl methacrylate (MMA; Stage I), then 20% styrene (Sty; Stage II). The resulting modifier polymer is blended with a MMA/styrene/ethyl acrylate matrix copolymer at a level to give 15 wt. % butadiene based on the total blend weight, which blend gave a sample having a notched Izod impact of 1.6 (ft-lb/in.), an ASTM have value of 11%, and an ASTM yellowness index of 13.0. These results indicate that modest impact was achieved, but optical properties were still far from optimum. Similarly, J. Schmitt and R. Quinn (U.S. Pat. No. 4,242,469) disclose compositions of acrylic/vinyl aromatic copolymers containing a mixed impact modifier blend containing a Bd core with at least two different levels of "grafted outerstage." This combination gave improved impact properties, but at the expense of optical properties which were even poorer than in the '256 disclosure.
A. Berzinis and W. Wills, in European Patent Application 265,142, disclose core-shell modifiers useful in styrenic polymers and copolymers. The Berzinis/Wills modifiers have lightly crosslinked (high swell index) rubber cores, at least one hard polymer shell of defined compositions, and are in the form of particles having an average diameter less than 250 nm and the particles have a toluene swell index of from 13 to 45. However, the polymer blends prepared from these broadly disclosed modifiers are of much reduced clarity and without the optimum impact levels discovered in the polymer blends of this invention.
Thus, past methods for the impact modification of vinyl aromatic homopolymers and copolymers have one or more disadvantages. One of the objects of this invention is to provide polymer blends of an acrylic/vinyl aromatic matrix polymer with an impact-modifying rubber particle having an average particle diameter smaller than about 400 nanometers, which blends are substantially clear. It is a further object to provide blends having good light transmittance, low light scattering, and low intrinsic yellowness (in the absence of toners, dyes, or optical brighteners), without the need for other modification, such as heterogenous modifier outerstages or mixtures of different outerstage levels. It is a further object of this invention to give a useful level of impact modification without requiring the use of a nitrile-containing comonomer in either the matrix or modifier polymer. Another object is to provide polymer blends having, where desired, increased degrees of heat resistance and distortion resistance while achieving one or more of the former objects.